Pneumatic control of fuel for a twin spool jet engine



J. M. EMBREE 2,984,977 PNEUMATIO CONTROL OF FUEL FOR A TWIN sPOOL JET ENGINE May 23, 1961 Filed NOV. 18, 195'? nl ll xllllkmmwHHMrlllll: J M

FUEL TIQNK NVENTOR JHN M EMB/95E ATTORNEY United States Patent O i PNEUMATIC CONTROL OF FUEL FOR A TWIN SPOOL JET ENGINE John M. Embree, Farmington, Conn., assignor to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware 3 Claims. (Cl. Gli-39.28)

(Filed under Rule 47(b) and 35 U.S.C. 118) This invention relates to gas turbine power plants, more particularly to fuel controls for twin spool jet engines.

Present fuel controls for jet engines can be divided into two basic types; the hydromechanical type and the electronic type. 'Ihe electronic type in reality is a combination electronic-hydromechanical control. Both types of controls have a temperature limitation, which limitation is becoming increasingly important as the temperatures associated with a jet engine are increased through the use of better materials which permit operation at higher temperature levels. In the hydromechanical control it is common to use fuel as a modifying or actuating iluid for the speed governors, servo systems, pilot valves, feed back and other devices, and the attaining of higher temperature levels in jet engine operation has made it imperative that such use be discontinued. In the electronic control the life span of the thermocouples, amplifiers, relays, switches, solenoids and other electrical components used in a control is substantially reduced by operation in elevated temperature environments.

In the belief that pneumatics offers a range of `control functions with relative economy, interchangeability, durability and versatility as compared with hydromechanical and electronic controls, I have invented an all pneumatic fuel control for a twin spool jet engine. Such a control will overcome the temperature limitations of present controls. Further, leakage problems are less serious due to the reduced pressures at which the vcontrol elements will be operating. Also, a saving in weight may be realized through the use of air as the working uid instead of fuel because lower operating pressure levels permit the use of thin-walled components and because control functions are generally obtained by means of simpler mechanical components.

An object of my invention, therefore, is to provide an improved fuel control for a jet engine.

Another object of my invention is to provide an -all pneumatic fuel control for a twin spool jet engine.

Still another object of my invention is to provide -a jet engine fuel control in which all of the various control functions are produced pneumatically.

Other objects and advantages will be apparent from the following specification and claims, and from the accompanying drawing which illustrates an embodiment of the invention.

In the drawing:

The single ligure shows a twin spool jet engine in cornbination with an all pneumatic fuel control according to my invention.

Referring to the drawing in detail, indicates a twin spool jet engine having inlet 12, low pressure compressor 14, high pressure compressor 16, burner 18, high pressure turbine 20, low pressure turbine 22 and exhaust nozzle 24, in the direction of gas flow through the engine. Low pressure turbine 22 is drivingly connected to low pressure compressor 14 by shaft 26, and high pressure turbine 20 is drivingly connected to high pressure compressor 16 2,984,977 Patented May 23, 1961 ICC by `sleeve 28 surrounding shaft 26. The two turbinecompressor units or spools are not connected together, but rotate independently of each other. The structural arrangement is similar to well known twin spool jet engines.

Fuel for the engine is supplied from tank 30, being pumped `by pump 32 through conduit 34 to metering valve 36 where the quantity of fuel flowing to the engine is metered in accordance with predetermined relationships. Metered fuel ows to the engine from valve 36 through conduit 38 and ring manifold 40 which distributes fuel to the various combustion cans 42 in burner 18.

Metering valve 36 is of standard construction and includes an element which is both translated and rotated to vary effective Valve area and thus meter fuel iiow to the power plant. The valve comprises a cylindrical casing 44 having an annular groove 46 therein which communicates with conduit 34. Sleeve 48 is xedly mounted within the bore of the chamber and has a plurality of ports 50 therein, the ports being located in alignment with groove 46 to provide communication therebetween. Movable piston 52 is mounted within sleeve 48, the piston having one or more ports 54 substantially in alignment with ports 5t). The piston is both translated and rotated in response to various conditions or parameters of engine operation in a manner to be described below. Axial or rotational movement of the piston will vary the effective metering area of ports 54 and cooperating ports 50 to increase or decrease fuel flow to the engine. Fuel flows from conduit 34 into groove 46, through the opening defined by ports 50 and 54 and then through conduit 38 to the engine.

The pressure ydrop across metering valve 36 is regulated by being held constant in order that fuel ow to the engine will be a function solely of the effective area of metering ports 50 and 54. 'Ihe structure is conventional and includes piston 56 the upper end of which is subject by means of conduit 58 connected to conduit 34 to the pressure of fuel entering metering Yvalve 36, and the lower end of which is `subject by means of conduit 60 to the pressure of the fuel leaving metering valve '36 and to the force of spring 62. The force balance across piston S6 determines the size of orifice 64 and thus the by-pass flow from conduit 34 through conduit 66 to the inlet of pump 32.

Movable piston 52 in the metering valve is rotated to vary effective valve area as a function of compressor discharge pressure absolute, this pressure sometimes being referred to as burner pressure. Pressure station 68 is located in jet engine 10 downstream of high pressure cornpressor rotor 16 Where it is subject to the pressure of the gases entering burner 18. The station is connected by conduit 70 to various branch conduits which in turn are connected to pneumatic devices as will be explained. Branch conduit 72 is connected to the interior of bellows 74 which has one end iixedly connected to control casing 76. Evactuated bellows 78 is mounted on the casing directly opposite bellows 74 and the adjacent free end of each bellows is joined by rod `80. The midpoint of rod 80 is pivotably connected to balancing :arm 82 which rotates about pivot `84. Expansion or contraction of compressor discharge pressure responsive bellows 74 rotates arm 82 about pivot 84 to vary the flow of pressure air from nozzle `86, the distance of surface 87 on the ann from the nozzle determining the effective nozzle area and thus the flow of air from the nozzle. Nozzle 86 is supplied with air from conduit 70 through branch conduit 88 and restriction 90.

A simple pneumatic amplifier 92 is connected to branch conduit 88 and responds to variations of air ow through nozzle 8,6 to control a pneumatic postioning servo 94 which is connected to and rotates piston 52 in the metering valve. The servo amplijier is not essential to the operation of the control but is included to improve the speed of response of the positioning servo to pressure variations and to minimize the consumption of air by the control. The amplifier includes casing 96 having chamber 98 at one end in which is mounted bellows 110. The interior of the bellows is connected by conduit 112 to branch Iconduit 88 downstream of restriction 90. The ree end of the bellows is connected to pilot valve 114 having spaced grooves 116 and 118 about its surface. Conduit 120 connects groove 116 to branch conduit S8 upstream of restriction 90 and passage 122 connects groove 118 to drain. Another conduit, conduit 124 is located opposite land 126 on the pilot valve between grooves 116 and 118 and is connected to positioning servo 94. In the equilibrium position of the pilot valve conduit 120 and passage 122 are directly opposite grooves 116 and 118, respectively, and conduit 124 is opposite land 126. As should be obvious, expansion or contraction of bellows 110 in response to variations in the ow of air through nozzle 86 translates pilot valve 114 to connect conduit 124 and the positioning servo to either pressure `conduit 120 through groove 116 or to drain passage 122 through groove 118. Conduit 124 is connected by passage 128 to chamber 98 so that the pressure surrounding bellows 110 is the same as that in conduit 124, and the conduit also is connected by passage 130 to the end of pilot valve 1'14 opposite bellows 110 to balance the arrangement and provide a return force for the pilot valve.

Positioning servo 94 includes chamber 132 Within control casing 76. Bellows 134 is mounted within the chamber and the connection of conduit 124 with the servo is such that the exterior of the bellows is subject to the pressure in the conduit. Plate v136 covers the free end of bellows 134, the plate having rod 138 connected thereto and extending in both directions therefrom. The upper extension of the rod passes through opening 140 in boss 1-42 on balancing arm 82 and also through retaining bearing 144 in casing 76. The bearing is designed to permit sliding motion of the rod. The lower extension of rod 1'38 terminates in rack 146 which meshes with pinion 148 on shaft 150 connected to metering piston 52.

Two springs are coaxially mounted within bellows 134 surrounding rod 138. Outer spring 152 is mounted between casing 76 and plate 136 and supplies the principal returning force to the bellows, and inner spring 154 mounted between boss 142 on balancing arm 82 and plate 136 is a linear calibrated spring whose function is to produce a force on the balancing arm proportional to the position of rod 138 and end plate 136. Inasmuch as the displacement of balancing arm 82 near nozzle 86 occurs over a very small range and because the pivot 84 of the balancing arm may be of the ball bearing type having minimum friction, the position of rack 146 is closely proportional to compressor discharge pressure. Thus, variations in the engine pressure sensed by pressure station 68 cause expansion or contraction of bellows 74 to rotate balancing arm 82 and vary the flow of air through nozzle 86. This in turn varies the pressure in chamber 132 acting on bellows 134 to expand or contract the bellows, translate rack 146, and rotate metering piston 52 to vary the effective area of metering ports 50 and 54 accordingly.

Translation of piston 52 in metering valve 36 is controlled during steady state operation of power plant by speed setting cam `156, and during acceleration of the power plant by acceleration limiting cam 158. Each cam is a three-dimensional cam. Speed setting cam 156 is rotated as a function of pilots power lever position and translated as a Ifunction of compressor inlet temperature, and acceleration limiting cam 158 is rotated asa function of the speed of the high pressure comprsorturbine spool and translated as a function of compressor inlet temperature. The operation of these cams will be described indetail below.

Compressor inlet temperature input to cams 156 and 158 is obtained from temperature sensing bulb 168 mounted in engine inlet 12. The bulb is connected by line 162 to temperature responsive bellows 164. Expansion and contraction of the bellows in response to temperature changes operates through pneumatic amplifier 166 and pneumatic positioning servo 168 similar to pneumatic amplifier 92 and pneumatic positioning servo 94, respectively, to translate shaft 170 on which the two cams are mounted. Pneumatic amplifier 1166 is constructed like pneumatic amplifier 92, but has not been illustrated in detail to avoid further complicating the drawing.

The free end of temperature responsive bellows 164 is connected to balancing arm 172 which rotates about pivot 174 to Vary the iiow of pressure air from nozzle 176. Pressure air for the nozzle is supplied by conduit 7 ii through restriction 178, and the flow of air from the nozzle determines through pneumatic amplifier 166 the pressure in conduit and chamber 182 which acts on bellows 184 within positioning servo 168. Spring 186 inside bellows 184 supplies the principal return force to the bellows and spring 188 within the bellows is a linear calibrated spring producing a force on balancing arm 172 proportional to the position of shaft 170 which is connected to plate 190 on the free end of bellows 184. Thus, a change in the temperature of the air entering the inlet of engine 10 is reilected by expansion or contraction of bellows 164 to change the position of balancing arm 172, vary the flow of air from nozzle i176 and the pressure in chamber 182, and expand or contract bellows 184 to translate shaft 170 and cams 156 and 158 thereon; spring 188 feeding back the translated motion to the balancing arm to restore equilibrium inthe system.

Speed setting cam 156 is rotated when power lever 192 is moved by the engine operator, the lever being connected through appropriate linkage and rack 194 to pinion 196 on shaft 170. A suitable coupling 198 is provided along shaft 170 to permit rotation of that section upon which cam 156 is mounted when the power lever is actuated.

Rotation or translation of speed setting cam 156 actuates a pneumatic system to translate piston 52 in metering valve 36. The pneumatic system includes follower 210 held against the surface of cam 156 by spring 212. The follower is integrally connected to flapper valve 214 to control the flow of pressure air from nozzle 216. The follower-napper valve unit is pivoted at 218 so that as follower 210 is moved by a change in the position of cam 156 the iiapper valve rotates about pivot 218 to cause an increase or decrease in air iiow from the nozzle. Pressure air for the nozzle is supplied from conduit 79 by branch conduit 220 through restriction 222, and the flow of air from the nozzle determines through pneumatic amplier 224 the pressure acting on bellows 226 in chamber 228 Within pneumatic positioning servo 230. The positioning servo is connected to the pneumatic amplifier by conduit 232. The free end of bellows 226 has plate 234 thereon to which is attached rod 236. The opposite end of the rod terminates in support 238 for the follower-flapper valve unit 210-214 and nozzle 216. A flexible conduit 240 connects nozzle 216 to conduit 228 and pneumatic amplier 224 to permit relative motion of the rod supported elements.

Like the bellows within pneumatic positioning servos 94 and 168, bellows 226 within positioning servo 230 contains two springs; return spring 242 and linear calibrated spring 244. The linear spring is interposed between plate 234 and boss 246 on lever 248 which rotates about pivot 250 on casing 76. This spring is designed to apply a force proportional to the radius of the circumferential cam on cam 156 to an adjustable lever system. The adjustable lever system includes in addition to lever 248, balancing arm 252 4having trolley 254 at its upper end yand boss 256 at its lower end. The balancing arm is fulcrumed at pivot 258 on casing 76. By virtue of thumbscrew adjustment 260, the position of trolley 254 on balancing arm 25.2 may be adjusted with respect to rotatable lever 248 to vary the relationship between the balancing arm and the lever.

The adjustable lever system applies a speed setting force to balancing arm 252 to control the flow of pressure air through nozzle 262. Pressure air for the nozzle is supplied from conduit 70 by branch conduit 264 through restriction 266, and the flow of air from the nozzle determines through pneumatic amplier 268 the pressure in conduit 270 and chamber 272 acting on bellows 274 within pneumatic positioning servo 276. The free end of bellows 274 has plate 278 thereon to which is attached an extension of shaft 150 connected to movable piston 52 in metering valve 36. Coupling 280 in the shaft permits rotational movement of that part of the shaft driven by the compressor discharge pressure responsive system With respect to the part of the shaft connected to positioning servo 276. Thus, the positioning servo translates piston 52 in response to rotation of balancing arm 252 to meter fuel flow to the engine accordingly.

The force applied to balancing arm 252 by cam 156, primarily through a power lever or speed setting input to the cam but also through a compressor inlet temperature input, is opposed by a force produced by a pneumatic system responsive to the speed of the high pressure compressor. Speed sensing device 282 is connected to and driven by the high pressure spool in engine .-10. The connection includes bevel gear 284 mounted on sleeve 28 joining the high pressure compressor and turbine, the gear meshing with gear shaft 286 which in turn drives shaft 288 in the speed sensing device. Disc 290 is integrally connected with shaft 288 and has at least one pair of centrifugal ilyweights 292 mounted thereon. The force produced by the flyweights as a result of rotation of disc 290, which yforce is proportional to the square of the speed of the high pressure spool, is applied to plate 294 which controls the ow of pressure air from nozzle 296. The air from the nozzle is discharged into a chamber within inner bellows 298 connected to plate 294 and is vented from the chamber by vent 310. Nozzle 296 is supplied with air from conduit 70 through branch conduit 312 and restriction 314, and the flow of air from the nozzle determines through pneumatic ampliiier 316 the pressure in conduit 318 and pressure responsive bellows 320 connected to balancing arm 252. `Conduit 318 is connected by branch conduit 322 to chamber 324 dened between inner bellows 298 and outer bellows 326 in the speed sensing device to apply a balancing force to plate 294 and centrifugal yweights 292. Bellows 320 may be opposed by bellows 328 which can respond to any desired engine operating characteristic and appropriately bias the speed signal received by bellows 320. This opposing force is needed when the fuel control is used as a turboprop engine fuel control but is not essential when the control is used as a jet engine fuel control.

Pressure responsive bellows 320 is a speed sensing signal and tends to rotate balancing arm 252 in a clockwise direction in opposition to the speed setting signal imparted to the arm by pnuematic positioning servo 230. The difference between the moments of the speed setting force and the speed sensing force will be a moment proportional to the speed error of jet engine 10. This moment is opposed by the force on inner spring 330 in pneumatic positioning servo 276. The spring is interpositioned between plate 278 on bellows 274 and boss 256 on balancing arm 252. The force translates the plate and shaft 150 connected thereto until the force on inner spring 33t) rotates balancing arm 252 to a position where equilibrium is restored in the nozzle circuit of pneumatic amplifier 268. Outer spring 332 within the bellows supplies the principal return force to the bellows. At equilibrium the resulting pressure in chamber 272 of pneumatic positioning servo 276 pushes shaft 150 to a position proportional to the thigh pressure compressor speed error and thus controls fuel flow to jet engine accordingly. It thus can be seen that the gradient of inner spring 330 establishes the slope of the governor droop line for the control.

During acceleration of the jet engine translation to the left of piston 52 in metering valve 36 to increase fuel flow is controlled by acceleration limiting cam 158. The rotational position of the cam is a function of the speed of the high pressure Icompressor while the axial position of the cam is -a function of compressor inlet temperature. The temperature response yof the cam has been described above. The speed response of the cam is obtained from speed sensing device 282 and its associated pneumatic circuit. Conduit 318 in the pneumatic circuit is connected by branch conduit 334 to bellows 336. The free end of the bellows is connected to balancing arm 338, fulcrumed at pivot 340, which varies the flow of pressure air from nozzle 342. The nozzle is supplied with air from conduit 70 thorugh branch conduit 344 and restriction 346, and the flow of air from the nozzle determines through pneumatic amplier 348 the pressure in chamber 350 acting on bellows 352 within pneumatic positioning servo 354.

Plate 356 covers the free end of bellows 352 and has rod 358 extending from the center thereof. The rod extends through boss 369 on bal-ancing arm 338 and terminates in rack 362 which meshes with pinion 364 on cam shaft 170. Coupling 366 permits relative rotational movement of that section of shaft 170 on which cam 158 is mounted so that expansion and contraction of bellows 352 in response to variations in high pressure spool speed can rotate cam 158 accordingly. Spring 368 within bellows 352 supplies the return force to the bellows and inner spring 370 is a linear calibrated spring producing a force on balancing arm 338 proportional to the position of end plate 356.

Follower 372 is pivotably mounted on support 374 connected to lthe end of shaft opposite piston 52. The follower is intended to engage the surface of cam 158 and limit the maximum opening of metering valve 36 during acceleration of engine 10. As shaft 150 and piston 52 are translated to lthe left as the result of an -action such as movement of power lever 192 .to increase fuel ow to the engine, follower 372 strikes the surface of the cam. Continued movement of the shaft and piston to the left causes the follower and integrally connected apper valve 376 to rotate in a clockwise direction about pivot 378 on the support, against the force of spring 380, to restrict the ow of pressure air from nozzle 382 mounted on the support. Air for the nozzle is supplied from conduit 70 by branch conduits 264 and 384 through restriction 386 and exible conduit 388. The pressure air also is supplied to bellows 390 to provide a force assisting the speed sense force in bellows 329. When apper valve 376 restricts nozzle 382, a pressure build up in bellows 390 occurs which may expand 4the bellows to the point where abutment 392 on the free end of the bellows contacts the face of balancing arm 252. This force assists the force in high pressure spool speed responsive bellows 320 to increase the ow of air from nozzle 262, decrease the pressure in chamber 272 and permit shaft 150 and piston 52 to move to the right. The follower will continue to ride cam 158 until the pressure in bellows 320 increases in response to engne acceleration to the newly selected or required speed.

Expansion of bellows 390 is opposed by a spring loaded stop 394 which contracts extension 396 on abutment 392 to offset any initial pressure in the pneumatic system of nozzle 382. The spring load on the stop may be adjusted by means of screw 398. Further, there is a mechanical dead zone included in the operation of bellows 390 to allow balancing arm 252 to position itself independently when operating on the governor droop line. The dead zone is provided by undercut 410 on extension 396 where the extension passes through opening 412 in balancing 'arm 252.

A limiter responsive to the speed of the low pressure compressor-turbine spool in the engine is provided in the control to prevent low pressure spool speed from exceeding a preselected value. This limiter is incorporated in the control as a bias to the compressor discharge pressure responsive signal, which signal rotates piston 52 in metering valve 36. Speed sensing device 414 is connected to and driven by the low pressure spool. The connection includes bevel gear 416 at `the forward end of low pressure 14, the gear meshing with gear shaft 418 which in turn drives shaft 420 in the speed sensing device. Disc 422 is integrally connected with shaft 428 and has at ieast one pair of centrifugal ilyweights 424 mounted thereon. The force produced by the yweights as a result of rotation of disc 422, which force may be proportional to the square of the speed of the low pressure spool, is applied to plate 426 to control the iiow of pressure air from nozzle 42S. The air from the nozzle is discharged into chamber 438 within inner bellows 432 connected to plate 426 and is vented from the chamber by vent 434. Nozzle 428 is supplied with from conduit 70 through branch conduit 312 and restriction 436, and the flow of air from the nozzle determines through pneumatic amplifier 438 the pressure in conduit 440 and pressure responsive bellows 442 connected to balancing arm 82 in the compressor discharge pressure responsive pneumatic servo system. Conduit 440 is connected by branch conduit 444 to chamber 446 defined between inner bellows 432 and outer bellows 448 in fthe speed sensing device to apply a balancing force to plate 426 and centrifugal flyweights 424.

Low pressure spool speed responsive bellows 442 is opposed by spring loaded stop 450 which establishes the speed at which the bias becomes effective. The adjustment of the stop is achieved by means of screw 452. When low pressure spool speed exceeds the preselected value the bellows 442 expands to rotate balancing arm 82 in a clockwise direction about pivot 84 and relieve the pressure in chamber 132 in pneumatic positioning servo 94. The reduction in pressure permits downward movement of rod 138 and rack 146 to rotate piston 52 in metering valve 36 in a reverse direction to the direction produced by increasing compressor discharge pressure, thus 4causing a reduction in fuel iioW to engine 10.

The ilyweights in speed sensing device 282 and 414 may preferably be designed so that the force on plates 294 and 426, respectively, is built up proportional to the square of high pressure spool speed and low pressure spool speed. However, the flyweights may be designed so that through the governing range the change in output pressures are approximately proportional to the change in speeds.

Minimum fuel ilow to engine is provided by passage 454 in metering valve casing 44, the passage connecting annular groove 46 and a downstream portion of the metering valve structure to bypass metering ports 50 and 54. Minimum fuel flow for deceleration may be provided by another orifice pair in metering valve 36.

Pneumatic amplifier 92 and pneumatic positioning servo 94 are illustrated :and described in greater detail than the other amplifiers and positioning servos. This has been done to avoid repetition `and further complicating the disclosure. However, it is to be noted that amplifiers 166, 224, 268, 316, 348 and 438 are substantially similar in construction to amplifier 92, and that positioning servos 168, 239, 276 and 354 are substantially similar to positioning servo 94.

Operation Operation of engine 10 is manually controlled by the pilot through power lever 192, movement of the power lever to a certain position establishing a speed and resultant thrust at which the engine will operate. Movement of the power lever rotates speed setting cam 156. This in turn moves follower 210 and ilapper valve 214 to actuate positioning servo 230, lever 248, balancing arm 252, nozzle 262 and positioning servo 276 to translate shaft 150 and piston 52 connected therewith and vary the area of ports 50, 54 in metering valve 36. When equilibrium has been established in the system through the effect of speed sensing device 282, nozzle 296 and bellows 320 on balancing arm 252, the resulting new position of the ports will permit a quantity of fuel suicient to maintain the selected high pressure spool speed to flow to the engine.'

If power lever 192 is advanced to a position calling for increased engine speed, shaft and piston 52 are driven to the left by the pneumatic systems to increase fuel ow to the engine until follower 372 contacts the surface of acceleration limiting cam 158 to temporarily halt the further increase of fuel flow. This cam is rotated as a function of the actual speed of the high pressure spool through speed sensing device 282, nozzle 296, bellows 336, nozzle 342 and positioning servo 354. Through apper valve 376, bellows 390 and balancing arm 252, the cam limits fuel iiow to prevent'the actual speed of the spool from entering the surge region during acceleration to the newly selected speed. Cam 158 is rotated as the actual spool speed increases to continuously vary the acceleration fuel flow limit, and at the same time the pressure in speed sensing bellows 320 is increasing due to the increase in spool speed. A point is reached at which equilibrium between the speed setting forces acting in a counterclockwise direction on balancing arm 252 and the speed sensing forces acting in a clockwise direction on the arm is reached and the tendency of metering piston 52 to translate to the left is terminated. Fuel ilow to the engine will then be the amount required to give the higher selected speed of high pressure compressor 16.

Cams 156 and 158 continuously are translated through temperature responsive bellows `164, nozzle 176 and positioning servo 168 by variations in compressor inlet temperature and, therefore, the position of speed setting cam 156 will be biased by any change in compressor inlet temperature. Similarly, the acceleration limit imposed upon the engine by cam 158 will be biased by a change in compressor inlet temperature. Through all phases of engine operation fuel flow to the engine is responsive by virtue of bellows 74, nozzle S6 and positioning servo 94 to variations in compressor discharge pressure. These variations serve to rotate metering piston 52 and vary the area of metering ports 50 and 54 with changes in the pressure.

Once steady state operation of the power plant has been established for a particular power lever setting, a predetermined thrust will be generated by the engine through control of the fuel ilow thereto as a function of engine speed, compressor inlet temperature and compressor discharge pressure. A maximum speed limiter for the low pressure spool is provided by speed sensing device 414, nozzle 428 and bellows 442 through its effect on the rotation of piston 52 lby the compressor discharge pressure responsive system.

It is to be understood that the invention is not limited to the specific embodiment herein illustrated and described, but may be used in other ways without departure from its spirit as defined by the following claims.

I claim:

1. For a twin spool jet engine having a rst compressor, a second compressor, a burner and a fuel supply to the burner, a `fuel control for controlling fuel flow to said burner and manual means for controlling said fuel control, said fuel control including valve means in said fuel supply, pneumatic means responsive to compressor discharge pressure for varying the area of said valve means, pneumatic means responsive to the speed of said second compressor for biasing said compressor discharge pressure responsive means, pneumatic means responsive to said manual means and compressor inlet temperature for varying the area of said valve means and pneumatic means responsive to compressor inlet temperature and the speed of said first compressor for limiting the maximum area of said valve means.

2. In combination with a twin spool jet engine having a rst compressor, a second compressor rotationally independent of said first compressor, a burner and a fuel supply, a fuel control for controlling the quantity of fuel supplied to said engine, said fuel control including valve means capable of rotational and translational movement for metering fuel, pneumatic servo motor means responsive to an engine pressure for imparting one of said movements to said valve means, means responsive to the speed of said first compressor for biasing said engine pressure responsive motor means, first three-dimension cam means, manual input means and engine temperature input means to said first cam means, pneumatic servo motor means actuated by said first cam means for imparting the other of said movements to said valve means, second three-dimension cam means, second compressor speed input means and engine temperature input means to said second cam means and pneumatic servo motor means actuated by said second cam means for limiting the travel of said valve means in one of said movements.

3. In combination with a jet engine having a compressor, a burner and a fuel supply to the burner, a fuel control for controlling the quantity of fue] supplied to said engine, said fuel control including valve means for metering fuel, pneumatic actuated means responsive to at least one parameter of engine operation for varying the area of said valve means, and means for limiting the maximum area of said valve means, said limiting means including adjustable stop means responsive to at least one parameter of engine operation, nozzle means through which an engine pressure is discharged, follower means adapted to contact said adjustable stop means, exible means defining a chamber, said chamber being connected to said nozzle means, flapper means operatively connected with said follower means for variably controlling the discharge of pressure through said nozzle means to establish a continuously variable pressure in said chamber according to the position of said apper, and means operatively connected with said flexible means for operating said valve means upon a build-up of pressure in said chamber.

References Cited in the le of this patent UNITED STATES PATENTS 1,830,636 Bragg et al. Nov. 3, 1931 1,992,048 Temple Feb. 19, 1935 2,588,678 Wills Mar. 11, 1952 2,759,549 Best Aug. 21, 1956 2,807,138 Torell Sept. 24, 1957 2,934,898 Graefe et al. May 3, 1960 2,939,280 Farkas June 7, 1960 

